Types of Metal Finishes: Contact Resistance & More Electronics Considerations

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Connectors are an often overlooked aspect of electronic design but are certainly essential if you want to interact with the world around you, charge a battery, or output a result. In audio connectors you always hear that the “gold” standard is gold coated connectors and it is for good reason. Gold contacts provide the best balance of resistance, durability, and cost in many situations. So gold may have center stage but it isn’t the only coating option out there and is not the best for every situation. Today we are going to look at the various coating options available and how they excel in different applications.

What are the Characteristics of Metals?

The connector finishes are typically separated out into two categories based on their characteristics. Characteristics are inherent to the metal so the materials are bucketed like they are on the periodic table and the names reflect that, Noble and non-Noble contact finishes. When you look closely at the options available you will start to see pretty clear differences. The things to look at are: resistance(measured in ohms), mating force(is the material self lubricating, does it respond well to a wipe type connection), oxidation and corrosion, mechanical wear, and response to various temperatures. Different coatings will have a different blend of these characteristics to suit your application. 

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Contact Resistance and Cycle Count

One specific characteristic I want to touch on is cycle count. Cycle count details the number of mating cycles a connector can go through before it no longer meets the specified contact resistance. The number of cycles is very much related to the mating surface coating material and thickness. The thicker the coating or the more resistant to wear and corrosion the longer a connector can remain within the specified contact resistance. Once you meet that cycle count it doesn’t mean the connector is done it just means that you might see a rise in the resistance. If you are able to handle higher contact resistance you can see a dramatic increase in the cycles that your connector can handle before it is considered failed.

Gold in Electronics: Noble Contact Finishes

Noble contact finishes use gold or palladium and their alloys as the contact surface. The finishes are thin electroplated coatings on top of a structure material to help improve durability and reduce costs since having the contact be completely gold would be very expensive and malleable meaning it could deform and reduce the mating force. Typical coating thickness for gold is between 0.4um and 0.8um on top of nickel or copper. The high corrosion resistance and lack of oxidation make noble finishes great for harsh environments and their performance leads to high cycle lifetimes for high reliability applications. A crimp connector like the 0330013004 from Molex is gold plated and can fit into a variety of connector housings.

Material Resistivity
ρ (Ω·m) at 20 °C

Conductivity
σ (S/m) at 20 °C

Temperature Coefficient (K−1)
Silver 1.59×10−8 6.30×107 0.0038
Copper 1.68×10−8 5.96×107 0.003862
Gold 2.44×10−8 4.10×107 0.0034
Nickel 6.99×10−8 1.43×107 0.006
Palladium 10.54×10−8 1×107 0.0042

Silver in Electronics: Non-Noble Contact Finishes

Non-noble contact finishes use tin or silver and their alloys as the contact surface. Now you might say “but silver is a noble metal” which is true but it has a bad habit of tarnishing in some environments, especially those with higher than normal amounts of sulfur, so it does not get the same prestige as gold and palladium. Silver also has a high coefficient of friction and poor wear characteristics resulting in high insertion force and poor durability. Tin also has an oxide layer issue but this can be mitigated by using a longer connector wipe which will scrape off oxidation and allow for better contact. One way to work past the insertion force issues on non-noble finishes is to use a connector lubricant. 

Tin in Electronics: Industrial & Commercial Use

Tin is often used in industrial and commercial environments where the connectors are not removed often. Silver and straight nickel finishes are seen on battery and power contacts respectively. Silver is popular in high current applications as well since it has the lowest resistivity of all the metals meaning you can push high currents through it with minimal voltage drop and heating. A good example of the current handling capabilities of silver is the Super Sabre system from Molex which can push 34A per blade on the connector.

Connector finishes have a very big impact on how your system will hold up to usage. There are a lot of options out there with varying characteristics. By putting some thought in at the beginning you can have the right connectors for your system to meet the spec’s you need.

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